1. Field of the Invention
This invention relates in general to liquid phase epitaxial (LPE) crystal growth and, in particular, to a method for controlling the deleterious effects of impurities on the electronic properties of crystals grown by LPE processes. This invention relates especially to a method for controlling the effects of residual oxygen on the electronic properties of semiconductors layers formed by LPE growth.
2. Description of Prior Art
Liquid phase epitaxial deposition is one of the principal methods used to manufacture compound semiconductors such as those in which an element of group III of the periodic table is reacted with an element of group V or an element of group II is reacted with an element of group VI. These compound semiconductors often have properties which make them attractive for use in electronic devices, particularly those which operate at high frequencies. However, the consistent preparation of high quality active layers has posed a serious limitation for use of these semiconductor compounds.
One of the most persistent problems has been the deoxygenation procedures that are used in the preparation of crystal layers by LPE growth. The importance of controlling oxygen levels in LPE growth has been well documented. Consider, for example, the III-V compound semiconductor gallium arsenide (GaAs), which has the unusual combination of a large band gap and a high electron mobility which make it particularly attractive for high frequency devices. There are reports that defects in GaAs crystals are substantially increased if a few parts per million (ppm) of oxygen are present in the LPE growth ambient and that there is a decrease in carrier mobility with increasing oxygen in the growth ambient. Most prior procedures for LPE growth have utilized an extended hydrogen anneal (approximately 20 hours) in the temperature range 600.degree.-900.degree. C. of the liquid gallium solvent prior to growth for the purpose of removing oxygen (and possibly sulfur) from the liquid gallium solvent. Properties, such as the carrier density, the carrier type, and the mobility, are profoundly influenced by the time and temperature of the anneal as well as the nature of the liquid gallium ambient during the anneal--for example, the purity of the hydrogen protective atmosphere and the materials in contact with the liquid gallium and hydrogen.
There is documentation of changes from n- to p-type conduction with increasing pregrowth annealing temperatures and of optimum annealing times in order to attain the highest mobilities. These observations suggest that there are beneficial and detrimental aspects of the pregrowth hydrogen anneal; the beneficial aspect concerns the removal of oxygen, a donor, from the gallium melt but with a concurrent contamination with C or Si acceptors. The existence of C- and Si-containing species at the ppm level in purified hydrogen has been confirmed using molecular beam mass spectrometry. During the long times required for the deoxygenation of the gallium melt with hydrogen, it is probable that C and Si acceptor impurities are incorporated as the oxygen donor is removed, thus explaining the n- to p-type conversion of the specimen.
At least partly because of this variable and unpredictable interaction of the ambient with the condensed phase of the system during the prolonged pregrowth anneal, the prior art methods of LPE growth have not been entirely satisfactory in consistently providing high quality semiconductor layers.